This invention relates generally to a position measuring device and particularly, to a photoelectric angle measuring device for measuring the angular position of two relatively rotatable objects.
Such position measuring devices are used, for example, in processing machines for measuring the relative position between a tool and workpiece. Typically, a scanning unit connected to the tool scans a graduation plate connected to the workpiece. The signal obtained as a result of scanning is used to determine the angular position of the tool with respect to the workpiece.
A previous device directed at measuring the position of two relatively rotatable objects is disclosed in U.S. Pat. No. 4,792,678 for "Photoelectric Angle Measuring Device" by Alfons Spies, issued Dec. 20, 1988, assigned to the assignee of the present invention, and incorporated herein by reference. The '678 patent discloses an invention for angle measuring in which the eccentricity error is compensated by diametrical scanning of a grid graduation. The '678 invention includes a scanning unit to which one of the objects is attached and a rotatable graduation support to which the other object is attached. In one of the embodiments of '678 invention, two linear graduations (G1a and G2a in FIG. 3) are used for the deflection of the positive and negative diffraction beams. In this embodiment of the '678 invention, the scanning unit is comprised of a light source, the two linear grids, and a photoelement. The graduation support may be a graduation cylinder or a graduation plate. The graduation support includes a grid graduation having first and second graduation zones. A beam of light from the light source is diffracted by the first graduation zone resulting in beams of a positive and negative first orders of diffraction. These beams are then directed through the two linear grids and brought into interference at the second graduation zone which is located diametrically opposite the first graduation zone on the grid graduation. The photoelement then measures the combined interfering beams and produces a signal that can be appropriately processed. Evaluation of the interference of the beams of the positive and negative first orders of diffraction can made to compensate for eccentricity error.
The present invention is directed to an improved position measuring device which provides high resolution angular measurement.
Accordingly, it is an object of the present invention to create a simply constructed angle measuring device in which the evaluation of interfering diffraction beams leads to phase-displaced signals having a high degree of modulation.
It is another object of the invention to provide a device of relatively simple construction that produce pinpointedly phase-displaced signals.